Locomotives were classified by track gauge, motive power, function and power (or model number) in a four- or five-letter code. The first letter denotes the track gauge. The second letter denotes motive power (diesel or electric), and the third letter denotes use (goods, passenger, mixed or shunting). The fourth letter denotes a locomotive's chronological model number.

In 2002, a new classification system was adopted. For newer diesel locomotives, the fourth letter denotes their horsepower range. Not all diesel locomotives were reclassified, and the fourth letter continues to denotes their model number.

A locomotive may have a fifth letter, generally denoting a technical variant, subclass or subtype: a variation in the basic model or series, or a different motor or manufacturer. Under the new system, the fifth letter further refines horsepower in 100-hp increments: A for 100 hp, B for 200 hp, C for 300 hp and so on. A WDP-3A is a 3,100 hp (2,300 kW) locomotive, and a WDM-3F is 3,600 hp (2,700 kW).

The system does not apply to steam locomotives, which are no longer used on main lines. They retain their original class names, such as M- or WP-class.

Nineteenth-century railway companies ordered custom-built locomotives, usually from British manufacturers. The multiplicity of similar-but-not-identical designs increased manufacturing cost and slowed production. During the 1890s, British manufacturers were busy and Indian railway companies looked to Germany and the United States for locomotives.[2] Engines used were:

After acrimonious words in The Times and Parliament, the British Engineering Standards Committee (later the British Engineering Standards Association) began designing a series of locomotives for use by Indian railways. The first two designs emerged in 1903: a 4-4-0 passenger and 0-6-0 goods. They were revised in 1905 and 1906 with additional heavier, more-powerful locomotives:

These advisory BESA designs were customized by the railway companies, which used different classification systems; only the state-operated railways used the class designations SP, SG, PT, HP, AP, HG and HT. When superheating was accepted, superheated versions were classified SPS, SGS and so on (if built with superheaters) and SPC, SGC and so on (if converted from saturated to superheated).

Indian Railways locomotives: 1935 XP class (left) and 1927 XB class

After World War I, larger and more-powerful locomotives were designed by British consulting engineers for the Indian government. They began to appear in 1927:

During World War II, large numbers of 2-8-2 locomotives were acquired from the United States and Canada and classified AWD and CWD. The Baldwin Locomotive Works adapted the USATC S160 Class locomotive design for India, and it became class AWC. Sixty broad-gauge locomotives were built in 1944 as part of an order of 180 S160 engines. In addition to modified frame spreaders, axles, cylinders, and cab, the Indian locomotives had a turbo generator and electric lighting (not included in the standard European design). Many parts (including boilers) were identical to those in standard-gauge locomotives.[3]

(clockwise from upper left) WL- and YG-class locomotives; an M2-162, and a display engine in Guntur Division

Although new classes were designed shortly before the war, many did not enter service until the post-war period. The new classes were indicated by the change of broad-gauge prefix from X to W, and plans were implemented to begain manufacturing locomotives in India. The new classes were:

WDM 1: India's first mainline diesel electric locomotives, introduced in 1957 and imported from ALCO. Out of service, the 1,950-horsepower (1,450 kW) locos' top speed was 110 km/h (68 mph).

WDM-2: The country's most widely-used and first homemade mainline diesel-electric locomotives, the prototypes were made by ALCO. Introduced in 1962, over 2,700 have been manufactured and they are rated at 2,600 hp (1,900 kW).

WDM-2A, WDM-2B: WDM-2 variants; the WDM-2A has dual brakes, and the 2B usually has air brakes.

WDM-3: Eight were imported in 1970 from Henschel & Son. With a hydraulic transmission, the 2,500 hp (1,900 kW) engines have a maximum speed of 120 km/h (75 mph) and are currently unused.

WDM-3A: Formerly known as the WDM-2C, it is a WDM-2 variant unrelated to the WDM-3. With a maximum speed of 120 km/h (75 mph), it has 30,450 kgf of tractive effort. Manufactured since 1994, it is one of India's most heavily-used diesel locomotives.

WDM-3A R: Formerly the WDM-2, it is rebuilt with DBR[clarification needed] on a short hood and is unrelated to the WDM-3.

WDM-3B: Co-Cobogies with a top speed of 120 km/h (75 mph), they operate out of the Uttar Pradesh sheds. Twenty-three were built by DLW. Similar to the WDM-3D, they are rated at 3,100 hp (2,300 kW).[6]

WDM-3C, WDM-3D: Higher-powered versions of the WDM-3A, rated at 3,300 hp (2,500 kW). The WDM-3C is rebuilt from the WDM-2. The WDM-3C and WDM-3D have maximum speeds of 120 km/h (75 mph) and 160 km/h (99 mph), respectively.

WDM-3E: Reclassified as WDM-3D, it is restricted to freight at 105 km/h (65 mph). Eight 3,500 hp (2,600 kW) units were known to have been manufactured by DLW.

WDM-4: Entering service with the WDM-2, its 2,600 hp (1,900 kW) prototypes were designed by General Motors. Although it was considered superior to the WDM-2, General Motors did not agree to a technology transfer agreement.

WDM-6: Two were made, and exported to Sri Lanka. Rated at 1,350 hp (1,010 kW), their maximum speed was 75 km/h (47 mph) and they had 19,200 kgf of tractive effort and Bo-Bo bogies.

WDM-7: Fifteen 2,000 hp (1,500 kW) locos were built from June 1987 through 1989. Designed for branch-line duty, they are used primarily for shunting.

Passenger locomotives are:

WDP-1: With Bo-Bo bogies and weighing 80 tons, their top speed is 120 km/h (75 mph). The 12-cylinder, 2,300 hp (1,700 kW) engines were built by DLW in 1970 and operate out of the Vijayawada and Tughlakabad sheds.

WDP-2 (new class name WDP-3A: Entering service in 1998, the DLW loco has a maximum speed of 140 km/h (87 mph), 29.25 tons of tractive effort and is rated at 3,100 hp (2,300 kW).[7][8]

WDP-3: WDP-1 prototypes, they were designed in 1996 by DLW. The 2,300 hp (1,700 kW) locos have Co-Co bogies.

WDG-5: Developed by Diesel Locomotive Works and supported by Electro Motive Diesels, the first unit left DLW on 25 February 2012. Numbered upwards from #50001, two were produced by 29 April 2015. Rated at 5,500 hp (4,100 kW), it has a fire control system, a TFT display and driver's toilet. The locomotive has entered serial production, and is assigned to the Sabarmati Diesel Loco Shed.

WDS-1: India's first widely-deployed diesel locomotive (imported in 1944–45), the 386 hp (288 kW) engine is out of service.

WDS-2: With o-C-o[jargon] bogies and an eight-cylinder engine, it is based at Central Railway. Built by Kraus Maffei in 1954–55, the 440 hp (330 kW) engine has 15,420 kgf of tractive effort and a maximum speed of 54 km/h (34 mph).

WDS-3: The 618 hp (461 kW) locomotives of this class were rebuilt and reclassified as WDS-4C from 1976 to 1978. With 17,100 kgf of tractive effort, they were built in 1961.

WDS-4, -4A, -4B and -4D: Designed by Chittaranjan Locomotive Works (CLW), the 600–700 hp (450–520 kW) locos have C bogies and were built from 1968 to 1997.

WDS-4C: Rebuilt by CLW and reclassified from WDS-3, the 700 hp (520 kW) locomotive has 18,000 kgf of tractive effort and C bogies. Out of service, its maximum speed is 65 km/h (40 mph).

WDS-5: Some are used for industrial shunting, and a few are used on Indian Railways. It is rated at 1,065 hp (794 kW).

WDS-6: Heavy-haul shunters made in large numbers for industry and Indian Railways, it is rated at 1,200–1,350 hp (890–1,010 kW).

WDS-6R, -6SL and -6AD: Variants of the WDS-6, the -6SL is exported to Sri Lanka. The -6AD has a maximum speed of 50 km/h (31 mph) and a six-cylinder engine.[12]

WDS-8: Five 800 hp (600 kW) locos, with a maximum speed of 35 km/h (22 mph) and 22,000 kgf of tractive effort, were made by CLW and transferred to steelworks.

The WCDS6, a YDM4 locomotive, was converted to broad gauge by the Golden Rock Railway Workshop for large industrial companies; the first one was delivered to RITES. New water and air lines were added, the control stand was modified, and it has a dual brake system.

These locomotives, no longer in use, were used only in and around Mumbai with 1,500 V DC power. All sections of Central Railway, including Thane to Mumbai CST (Main Line), Nerul to Thane (Trans-Harbour Line), Vadala Road to Mahim (Harbour Line, connecting Central and Western Railway) and Panvel to Mumbai CST (Harbour Line) have been converted to 25 kV AC.

WCM 2: Twelve 3,120 hp (2,330 kW) locomotives with Co-Co bogies and a maximum speed of 120 km/h (75 mph) were built by Vulcan Foundry in 1956–57 and modified by the Research Design and Standards Organisation (RDSO).

WCM 3: Three 3,400 hp (2,500 kW) Co-Co locomotives with a maximum speed of 120 km/h (75 mph) were built by Hitachi in 1958, used in Kolkata and transferred to Mumbai.

WCM 5: Built in 1962 by the Chittaranjan locomotive works to RDSO specifications, with auxiliaries by Westinghouse and North Boyce, the 3,700 hp (2,800 kW) Co-Co engines were India's first indigenously-designed DC electric locomotives.

WCM 6: Two 4,700 hp (3,500 kW) locomotives were built in 1995 by CLW and converted to AC power. Initially rated at 105 km/h (65 mph), they are restricted to 65 km/h (40 mph).

WCP 3, WCP 4: The GIPR EB/1 and EC/1 were among the earliest electric locos used in India. One of each class was built by Hawthorn Leslie and Company in 1928 with a 2′Co2′ wheel arrangement.

Preserved 1928 WCG1

Goods locomotives were:

WCG 1: GIPR EF/1, 2,600–2,950 hp (1,940–2,200 kW) Swiss crocodile locomotives imported in 1928 (10 from SLM and 30 from Vulcan Foundry). They were among India's earliest electric locomotives. The first, the Sir Leslie Wilson, is preserved at the National Rail Museum.

WCG 2: Fifty-seven 4,200 hp (3,100 kW) locomotives with a maximum speed of 90 km/h (56 mph) and 35600 kgf of tractive effort were designed by the Chittaranjan locomotive works in 1970 and built until 1977.[14]. They were used extensively around 2000 in Mumbai.[15])

WAP-2: Four units, decommissioned in the late 1980s and similar to the WAM-2 and 3 with Flexicoil Mark-II bogies and 2,910 hp (2,170 kW)

WAP-3: Five WAP-1s have been rebuilt since 1987, with a maximum speed of 140 km/h (87 mph).

WAP4

WAP-4: Upgraded from the WAP-1 for heavier loads by CLW in 1994, the locomotive is rated 5,060 hp (3,770 kW) continuous and has a speed limit of 140 km/h (87 mph).

Vadodara WAP5

WAP-5: Imported in 1995 from Switzerland and used on express trains, the 6,000 hp (4,500 kW) maximum, 5,450 hp (4,060 kW) continuous locomotive with Bo-Bo bogies and a speed limit of 160 km/h (99 mph) has been built by CLW since 2000.

WAP-7: Same design as the WAG-9, with a modified gear ratio. The 6,350 hp (4,740 kW) maximum, 6,000 hp (4,500 kW) continuous locomotives with a speed limit of 140 km/h (87 mph) have been built by CLW since 2000.

Goods locomotives are:

WAG-1: Built between 1963 and 1966, India's first AC freight locomotive was rated 2,930 hp (2,180 kW) and had B-B bogies and a maximum speed of 80 km/h (50 mph). It was removed from service in 2002, and the WAG-1S is a variant.

WAG-2: Built in 1964–65, the 3,450 hp (2,570 kW) locomotive with B-B bogies and a maximum speed of 80 km/h (50 mph) is out of service. The WAP-2 is a variant.

WAG-3: Ten 3,590 hp (2,680 kW) locomotives with monomotor bogies, 30,000 kgf of tractive effort, over 6,000 tons of hauling capacity up to 70 km/h (43 mph) on level track and a maximum speed of 80 km/h (50 mph) were built in 1965. It is out of service. The WAG 3A is a variant.[16]

WAG-4: Built by CLW between 1966–71, the 3,590 hp (2,680 kW) locomotive with B-B bogies and a maximum speed of 80 km/h (50 mph) is out of service. The WAG-4A and D are variants.

WAG5HA

WAG-5: India's most successful electric locomotives were designed by CLW in 1984. More than 1,100 were manufactured, and the WAG-5A, -5B, -5D, -5E, -5H, -5HA, -5HB, -5HD, -5HE, -5PE and -5RH are variants.

WAG 8: Similar in appearance to the WCAM-2 and technically to the WCAM-3, the experimental locomotive built by BHEL in 1998 is out of service.

WAG9

WAG-9: rated at 6,350 hp (4,740 kW), it has the same design as the WAP-7 with a modified gear ratio and was designed by Adtranz, Switzerland.

WAG-9H, WAG-9i and WAG-9Hi: Variants of the WAG-9, with the -9H the heavier version. The WAG-9i is fitted with IGBT traction converters.[jargon] The WAG-9Hi is probably a combination of the WAG-9H and WAG-9i.[15]

These locomotives are used only in the Mumbai. Their main purpose is to prevent the need to change engines. Mixed locomotives are:

WCAM 1: Designed by Chittaranjan Locomotive Works, total 53 were built between 1975 and 1979. Decommissioned by Western Railway, it is the only locomotive with reverse pantographs.

WCAM 2/2P: Designed by BHEL, 20 were built in 1995–96 and tested at 135 km/h (84 mph) on AC.

WCAM 3:53 were designed by Bharat Heavy Electricals, rated at 4,600 hp (3,400 kW) on DC traction and 5,000 hp (3,700 kW) on AC. Owned by Central Railway, they are the most widely-used locomotive on the Mumbai-Pune section.